This invention relates to the use of lithium alloy as an anode material in rechargeable high energy density batteries.
The cycle life and thermal safety properties of lithium rechargeable batteries are dependent on the lithium anode. The lithium anode fails most often because of the formation of dendritic and mossy lithium on the anode surface upon charging. The dendrite penetrates the separator between the anode and the cathode, which causes electrical shorting, reducing the cycle life of the cell. Mossy lithium is composed of many tiny lithium particles. The total surface area of the mossy lithium is large and very active, providing a source of thermal instability. The mossy lithium initiates temperature increases, when some trigger reaction is applied, which leads to thermal instabilities of the cell.
Several approaches for overcoming these problems have been developed. Initially, cell separators were used to minimize the effects of dendrite growth. However, cell separators are only effective initially, for the dendrites will eventually penetrate the separators and establish electronic shorts.
Lithium-Aluminum high concentration alloys have been investigated to avoid the above mentioned problems. There are two methods to prepare Lithium-Aluminum high concentration alloys. One is a powder metallurgical method and the other is an electrochemical method. For example, U.S Pat. No. 4,002,492 describes a powder metallurgical method of preparing Lithium-Aluminum alloys containing from about 63% to about 93% Lithium on an atomic basis. In this U.S. patent the Lithium alloy is made by sintering, by which process it is impossible to make a ductile and thin Li alloy sheet. Similarly, Japanese patent Tokkai-Sho-63-146351 describes an electrochemical method of preparing Lithium-Aluminum alloys. In these alloys, however, loss of capacity is inevitable and they are too fragile and brittle to wind cylindrical cells.
Composite electrodes comprised of a lithium-lithium nitride alloy as disclosed in European Patent Application 88301752 have also been developed. However, this alloy is hard to work, which makes it very difficult to prepare lithium-lithium nitride alloy foil for small cells, such as AA size cells.
There are several patents which describe lithium anodes with metallic material. For example, U.S. Pat. No. 4,434,213 describes a lithium anode having an aluminum-magnesium cladding. U.S. Pat. Nos. 4,652,506 and 4,794,060 describe an anode with a thin metallic foil of aluminium, magnesium and/or tin. U.S. Pat. No. 4,626,335 discloses a lithium aluminum anode with metal binder particles of unalloyed iron, copper, nickel or manganese. The main advantage sought by adding metallic elements is to disperse the dendrite growth points so that the growth of the dendrite is prevented and penetration of the dendrite through a separator, which is one of the causes of anode deterioration, does not occur. However, the formation of dendrites is only slowed down and not prevented by these new alloys.
The present invention attempts to further improve the prevention of dendritic growth while maintaining the necessary mechanical properties of the material for easy working. The new lithium alloy anode, according to the present invention demonstrates excellent cell performance and thermal stability.